As the hydraulic circuit of the type including a pressure compensating valve, a circuit as shown in FIG. 1 is heretofore known, for example.
That is to say; a discharge conduit 2 of a hydraulic pump 1 is connected with an inlet of a pressure compensating valve 3 whose outlet is connected with an inlet of a directional control valve 4. The arrangement is made such that when the directional control valve 4 is changed from its neutral position A over either to a first pressurized fluid supply position B or to a second pressurized fluid supply position C the fluid under pressure discharged by the hydraulic pump 1 is supplied into hydraulic actuators 5.
The above-mentioned pressure compensating valve 3 is arranged to be urged by the fluid pressure applied to its first pressure receiving portion 6 in combination with the resilient force of a spring 7 to a position D where the area of opening thereof is kept maximum, and also urged by the fluid pressure applied to its second pressure receiving portion 8 to a position E where the area of the opening thereof is kept minimum. The first pressure receiving portion 6 is connected with a load pressure circuit 9 so that the fluid pressure on the outlet side of the directional control valve 4 is supplied through a load pressure detection circuit 10 formed within the control valve 4 into the first pressure receiving portion 6. Whilst, the second pressure receiving portion 8 is connected with the outlet side of the pressure compensating valve 3 so that the fluid pressure on the inlet side of the directional control valve 4 is supplied into the second pressure receiving portion 8. Thus, the pressure compensating valve 3 is rendered operative in response to the pressure drop of the fluid under pressure flowing through the directional control valve 4.
The above-mentioned directional control valve 4 is arranged to be changed from its neutral position A over either to a first pressurized fluid supply position B or to a second pressurized fluid supply position C when the pressurized fluid discharged by an auxiliary hydraulic pump 11 is supplied through a pilot fluid pressure change-over valve 12 either into a first pressure receiving portion 13 or into a second pressure receiving portion 14. When the directional control valve 4 is located at its neutral position A its pumping port 15 is disconnected and the load pressure circuit 9 is allowed to communicate with a fluid tank 16.
The above-mentioned hydraulic pump 1 is of a variable displacement type, and the angle of swash plate 17 thereof is changed over by the action of a servo-cylinder 18 whose small diameter chamber 19 is supplied directly with the discharge pressure of the hydraulic pump 1 and whose large diameter chamber 20 is supplied with the discharge pressure of the hydraulic pump 1 through a control valve 21. The control valve 21 is rendered operative in response to the pressure differential between the discharge pressure of the hydraulic pump 1 and the load pressure in the above-mentioned load pressure circuit 9. The hydraulic pump 1 is arranged such that the discharge pressure thereof is set to become higher than the load pressure by a value which, for example, corresponds to the resilient force of the spring 22, and even when the directional control valve 4 is located at its neutral position A where the outflow of the fluid under pressure discharged by the hydraulic pump 1 is blocked, the discharge pressure of the hydraulic pump 1 is prevented from becoming excessively high.
In such a hydraulic circuit, when the directional control valve 4 is located at its neutral position A the load pressure circuit 9 is connected through a passage 23 formed within the directional control valve 4 with the fluid tank 16 thus keeping the load pressure at zero so that the pressure compensating valve 3 is held by the action of the discharge pressure of the hydraulic pump 1 at a position E where the area of opening thereof is kept minimum.
For this reason, when the directional control valve 4 is changed from its neutral position over either to the first pressurized fluid supply position B or to the second pressurized fluid supply position C to supply the pressurized fluid on the outlet side of the directional control valve 4 into the load .pressure circuit 9, the fluid pressure in the second pressure receiving portion 8 of the pressure compensating valve 3 will rise after the pressurized fluid has flowed into the second pressure receiving portion 8 by an amount corresponding to the stroke volume thereof, and therefore it takes a time for the pressure rise, which causes a delay in operation of the pressure compensating valve 3, thereby deteriorating the response of the entire hydraulic circuit.